Influenza is both a major human pathogen, causing approximately 50,000 deaths per year in the United States and a well-studied model system for cell entry by enveloped viruses. Influenza enters cells via a process of membrane fusion, but despite extensive study the mechanism by which influenza hemagglutinin catalyzes this process is not well understood. Experimental mutagenesis has yielded much data on the functional requirements of influenza fusion proteins, but we have no robust theory that could have predicted these results. This proposal seeks to develop a robust understanding of fusion peptide mechanisms sufficient to predict such mutations to hemagglutinin. We also wish to establish which elements of influenza entry are common to all membrane fusion, as modifications to the lipid environment can also promote or block fusion, and which may be virus-specific. We are developing high-performance simulation methods to analyze membrane fusion; in this work, we will use these methods to predict how mutations to influenza fusion peptides alter the virus's ability to infect cells and to investigate how lipid composition changes interact with these mutations to control viral infectivity. Computational predictions will be evaluated against experimental assays performed in the laboratory of collaborator Steinhauer.